Search engines allow the user to enter keywords that are run against a database (most often created automatically, by "spiders" or "robots"). Based on a combination of criteria (established by the user and/or the search engine), the search engine retrieves WWW documents that match the keywords entered by the searcher.
While all search engines are intended to perform the same task, each goes about this task in a different way, which leads to sometimes amazingly different results. Factors that influence results include the size of the database, the frequency of updating, and the search capabilities. Search engines also differ in their search speed, the design of the search interface, the way in which they display results, and the amount of help they offer.
In most cases, search engines are best used to locate a specific piece of information, such as a known document, an image, or a computer program, rather than a general subject.
Examples of search engines include:
The growth in the number of search engines has led to the creation of "meta" search tools, often referred to as multi-threaded search engines. These search engines allow the user to search multiple databases simultaneously, via a single interface. While they do not offer the same level of control over the search interface and search logic as do individual search engines, most of the multi-threaded engines are very fast. Recently, the capabilities of meta-tools have been improved to include such useful features as the ability to sort results by site, by type of resource, or by domain, the ability to select which search engines to include, and the ability to modify results. These modifications have greatly increased the effectiveness and utility of the meta-tools.
Popular multi-threaded search engines include:
Subject guides are hierarchically organized indexes of subject categories that allow the Web searcher to browse through lists of Web sites by subject in search of relevant information. They are compiled and maintained by humans and many include a search engine for searching their own database.
Subject guide databases tend to be smaller than those of the search engines, which means that result lists tend to be smaller as well. However, there are other differences between search engines and subject guides that can lead to the latter producing more relevant results. For example, while a search engine typically indexes every page of a given Web site, a subject guide is more likely to provide a link only to the site's home page. Furthermore, because their maintenance includes human intervention, subject guides greatly reduce the probability of retrieving results out of context.
Because subject guides are arranged by category and because they usually return links to the top level of a web site rather than to individual pages, they lend themselves best to searching for information about a general subject, rather than for a specific piece of information.
Examples of subject guides include: